HaShem told Moshe Rabbeinu, "Gather the Yidden on Shabbos and teach them its halachos, so that future generations will learn from you to gather together in the shuls and study on Shabbos, and thereby My Name will be praised."
Thus it was that when the Yidden were about to enter Eretz Yisroel, the Torah complained to HaShem, "What will become of me? Your people will be preoccupied with working the land and will not dedicate enough time to me." HaShem reassured the Torah: "I have a match for you – Shabbos. On that day the Yidden will abstain from working, and will enter the beis midrash and study Torah."
In this spirit, Chazal say that Shabbos and Yom-Tov were given to the Yidden so that they would be able to learn Torah. This applies particularly to those who work during the week, and who can now devote considerable time to Torah study.
(ילקוט שמעוני ויקהל רמז תח, שוע"ר סי' ר"צ ס"ג וס"ה)
Speaking of the holiness of Shabbos, the Frierdiker Rebbe once said:
In the past, a chassidishe baalabos would be entirely different on Shabbos, at ease and unrushed. Before davening he would listen to a maamar being taught. Then came davening, at a more leisurely pace than usual, each individual at his own level. He did not hurry home to the kugel. Though he honored Shabbos with fine food, he knew that this was not the most important matter. Every person would work on himself with guidance from the local mashpia. Today, too, there are many who keep Shabbos – but what is with the kedusha of Shabbos?
(סה"ש תש"א ע' 53)
The Rebbe explains that Shabbos is called a day of rest – not because one sits idly, doing nothing, for surely on Shabbos one must toil in the service of HaShem. Rather, it means that on Shabbos one takes pleasure in this service, just as one derives pleasure from carrying a heavy sack of precious stones. This is a foretaste of the time of Moshiach, "a day of Shabbos and rest," when we will experience intense pleasure as we rise to great heights in our service of HaShem.
(התוועדויות תשמ"ג ח"ד ע' 1983)
